The world salt production has crossed 240 million ton per annum and more than 60% of the total salt produced is used for industrial applications, chlor-alkali and specialty chemical industries being the major consumers.
The salt produced from brine by solar evaporation is found to contain substantial impurities like calcium, magnesium, sulphate, bromide, iodide and other trace element impurities. The presence of such impurities may necessitate further purification of the harvested salt to make it suitable for its specific end use. Superior quality industrial grade salt with specified limits of calcium, magnesium, sulphate, bromide, iodide and other trace element is preferred by these industries as the use of such salt reduces the brine purification cost and effluent generation.
Solar salt is produced from seawater; sub-soil brine, lake brine and solution mining of rock salt. The salt rocks are, in fact, of marine origin. All these sources constitute a multi-component salt system with a number of ions in the dissolved state and it is reported that sea water is a store house of chemicals with more than 73 elements present in dissolved state. Hence direct recovery of any of these salts in its pure form from these sources poses a great problem. Salt produced from the above brines through the well known process of fractional crystallization by solar evaporation is invariably contaminated with impurities such as calcium, magnesium, sulphate, bromide, iodide and other trace elements. All these impurities are detrimental for industries where salt is used as a basic raw material. On the other hand, solar salt production from natural brines is most cost-effective and viable alternative. It is, therefore, important to devise means of making solar salt with minimum impurities while retaining the advantage of cost-effectiveness.
Impurities such as Ca and Mg can critically affect the membrane in chlor-alkali plants and their concentration has to be reduced to ppb levels in the brine employed. It is of utmost importance to minimise such impurities to the maximum extent possible so that the cost of the downstream purification and associated waste formation can be minimised.
Similarly, high levels of halide impurities such as bromide and iodide in salt can be damaging for certain industrial applications. For example, high levels of iodide, can adversely affect the membrane employed in chlor-alkali process whereas high level of bromide can lead to formation of chloro bromo methane during chloroform production.
Although NaCl can be purified to any extent desired, there is an associated cost which would increase the cost of the raw material. Thus it is of great importance to bring about purification in the most practical manner feasible, which is the object of the present invention.
Reference may be made to the paper entitled “Industrial Minerals, April 1996” by V. M. Sedivy, wherein the critical importance of salt purity and the deleterious effects of contaminants on various industries are described.
Reference may be made to the paper entitled “Primary Brine Treatment Operations” by D. Elliott presented at the 1999 Eltech Chlorine/Chlorate Seminar on Technology Bridge to the New Millenium, Ohio, 13 Sep. 1999, wherein the critical importance of salt purity and the deleterious effects of various contaminants including heavy metals on chlor-alkali manufacture are highlighted.
Reference may be made to U.S. Pat. No. 7,037,481 dated May 2, 2006, wherein, Becenel, Jr. reports the methods and installations for producing ultra pure sodium chloride salt crystals primarily for use in saturating depleted brine resulting from the electrolytic decomposition of saturated brine in chlor alkali membrane cells for the production of chlorine, caustic soda and hydrogen. This invention particularly relates to the production of ultra pure sodium chloride salt crystals by processing primary treated brine by first acidifying the primary treated brine, then stripping the carbonic acid produced by acidification as carbon dioxide, and then returning the brine to a pH of about 6 or higher which is sufficient for processing it in evaporation equipment where the ultra pure salt crystals are produced. The process described above not only involves various unit operations but it is a costly proposition with respect to energy involvement.
Reference may be made to GB764013A, dated 19 Dec., 1956 entitled “Improved Method of Preparing Sodium Chloride Brines of High Purity” by Albright and Wilson have claimed that sodium chloride brines low in calcium sulphate content are prepared by dissolving solid sodium chloride contaminated with calcium sulphate in water in presence of a polyphosphate soluble in brine in the concentration range of 50-100 ppm. It is claimed that the amount of calcium sulphate is further decreased by dissolving solid sodium chloride in the presence of both the poly phosphate and water-soluble alkaline earth metal compound such as calcium chloride or acetate or barium chloride up to 1% level. The drawbacks of this process are that it is less appropriate for solar salt production and more appropriate as a means of post-treatment of brine obtained by dissolving salt.
Reference may be made to U.S. Pat. No. 3,360,343 dated 26 Dec., 1967 Grant A. Mickelson, Cary et. al. describes a process for the production of high purity salt with highly reduced levels of Ca and Mg, the said process consisting of preparing a saturated solution of NaCl contaminated with Ca and Mg, evaporating the solution to recover salt with reduced levels of Ca and Mg, treating the salt so produced with dilute mineral acids to form a slurry and recovering un-dissolved purified salt with reduced levels of Ca and Mg from the said slurry. The main objective of the said patented process is to reduce the levels of Ca and Mg in solar salt and there is no mention about the removal of trace elements from salt. Moreover, the process involves multi phase steps like recovering solar salt, purifying the salt to lower down the Mg and Ca impurities, crushing the salt to specific micron level, treating with mineral acid and recovering salt from the slurry. All these steps make the process highly uneconomical.
Reference may be made to EP 1545733B1 (WO 2004/018068) dated Jan. 3, 2007 by Akzo Nobel N.V., wherein an evaporative salt crystallization process that produces pure salt is disclosed. The process utilizes saccharide or its derivative in an evaporative process occurring at room temperature. The main disadvantage of the said process is that the saccharide is used in about 5% (w/v) concentration which would increase the viscosity of the brine and slow down evaporation and also add significantly to cost. Moreover, the use of saccharide will also increase the organic carbon content which is not acceptable for chlor-alkali and specialty inorganic chemical industries.
Reference may be made to U.S. Pat. No. 4,026,676 dated 31 May, 1977 entitled “Process for producing salt having a reduced calcium sulfate content” by H. W. Fiedelman a process for the preparation of the cubic crystalline form of sodium chloride is described using either (1) a feed and bleed procedure comprising admixing an alkali metal phosphate with an aqueous solution of salt to increase the super saturation of calcium sulphate there in and evaporating the brine at an elevated temperature and reduced pressure to cause crystallization of pure salt and concomitantly bleeding brine from the chamber to the feed brine such as to maintain the calcium sulphate in the dissolved state and prevent its precipitation with salt or (2) subjecting the brine to solar evaporation to concentrate the same to the salt point, i.e. that point at which the salt will crystallize from the brine, adding an alkali metal polyphosphate to brine at this point to increase the super saturation of calcium sulphate there in and processing the brine for salt production following the conventional method. The process involves addition of costly chemicals at a very high dosage level and will prove to be highly uneconomical.
Reference may be made to U.S. Pat. No. 6,812,011 dated 2 Nov., 2004 entitled “Process for the Removal of calcium ions from the Brine by Marine Cyanobacteria” by CSIR claimed that common salt with reduced Ca impurity can be produced from sea/subsoil brine by mopping up Ca in the brine through certain types of calcium loving marine cyanobacteria. The drawback of this process is that it describes the method of reduction of Ca impurities and does not describe the reduction of remaining impurities such as magnesium, sulphate and trace elements.
Reference may be made to U.S. Pat. No. 7,037,481 dated 2 May, 2006 by United Brine Services Company describes a process for the production of ultra pure sodium chloride salt crystals primarily for use in saturating depleted brine resulting from the electrolytic decomposition of saturated brine of chlor-alkali membrane cells. This invention specifically relates to the production of ultra pure sodium chloride salt crystals by processing primary treated brine through its acidification and processing the acidified brine in evaporation equipment where ultra pure salt crystals are produced. Apart from involving complicated steps, the process is cumbersome and energy intensive. Moreover, the main objective of the above patented process is different from the objectives of the present innovation that is reduction of trace element impurities carried along with solar salt.
Reference may be made to research article: “Rain Washing of Common Salt Heaps” by M. P. Bhatt et al. (Salt Research and Industry 10 (2), 1974, p 13) it is reported that sea salt, as produced in solar pans containing 0.16-0.18% (w/w) Ca2+, 0.3-0.4% (w/w) Mg2+ and 0.70% (w/w) SO42−, when subjected to rain washing contains 0.21% (w/w) Ca2+, 0.06% (w/w) Mg2+ and 0.60% (w/w) SO42−. Although rain washing reduces Mg impurities to some extent, the Ca and SO4 impurities cannot be reduced from the harvested salt even by repeated washings. On the contrary, it is observed that the concentration of Ca increases after rain-washing. Moreover, washing of salt either through rain washing or using method of counter/concurrent washing of salt with saturated brine is effective in removal of superficial impurities. The trace element impurities present in lattice of the salt crystals cannot be eliminated by simple washing.
H. M. Patel, in his research article that appeared in the Proceedings of 6th International Symposium on Salt, Vol. 2 pp. 515-533, has disclosed that Ca and SO4 impurities in salt can be reduced using the difference in dissolution rate of NaCl and CaSO4. The main drawbacks of the process are that it employs unit operations like dissolver and chemical process reactor. It also requires addition of lime and soda for the removal of Mg and Ca and subsequent filtration of brine.
In the article “Washing of Strip Mined Rock and Solar Salt at Leslie Salt Corporation US” (Symposium on Salt-I, Vol. 1, the Northern Ohio Geological society Incorporation, Cleveland (1961), p 449-464), A Woodhill has reported that Ca, Mg and SO4 impurities in solar salt can be reduced by mechanical washing. The main disadvantage of the method is that there is a 15-20% loss of salt and the method requires high capital investment. Moreover, the maximum level of reduction of Ca is 70% and embedded impurities are difficult to remove.
Reference may be made to the research article: “Upgrading Solar Salt by Mechanical Washing” by M. P. Bhatt et al. appeared in the Proceedings of 7th International Symposium on Salt, Vol. 2 pp. 517-525, wherein it is reported that salt loss in bench scale washing of freshly harvested sea produced at Bhavnagar; freshly harvested salt produced from well brine at Kharaghoda and freshly harvested salt produced from stored well brine at Andhra Pradesh in mechanical washery were found from 15-21%.
Reference may be made to the article “Manufacture of Solar Salt by Series Feeding System” (Salt Research and Industry, 11, 1979, p 9) R. B. Bhatt et al. report that solar salt with reduced levels of Ca can be produced from sea water by a series feeding method wherein the salt is harvested in two stages i.e. between 25.5-27° Be′ (Sp. Gr. 1.214-1.230) and 27-29° Be′ (Sp. Gr. 1.230-1.250). Salt harvested in the first stage contains reduced levels of Mg and SO4 impurities whereas the salt harvested in the second stage contains low Ca whereas the Mg and SO4 impurities are comparatively higher. The drawback of this process is that calcium and sulphate impurities cannot be reduced beyond a certain point even though higher levels of reduction are desirable. The article does not describe the processing of sub-soil brines which is deficient in sulphate content as compared to sea brine. More over there is no mention about the eradication of trace element impurities from salt.
Reference may be made to the Indian Patent No. 191912 entitled “Preparation of Sodium Chloride Containing Low Ca2+ Impurity from Sea Brine in Solar Salt Works” by CSIR claimed that addition of a polysaccharides such as starch in a concentration of 50-150 ppm into concentrated brine can reduce calcium impurity in salt to less than 0.05-0.1 percent. The drawbacks of the process are that it requires addition of hot solution of starch which is both cumbersome and costly and addition has to be repeated several times. The process aims mainly on the reduction of Ca2+ and no mention is made of the effect of the treatment on other impurities in salt.
Reference may be made to WO 2004069371 dated 19 Aug., 2004 Kamishima Hiroshi et al. have claimed that sodium chloride crystals with reduced impurities can be produced from aqueous sodium chloride solutions by passing the solution through a column packed with an adsorbent on to which the impurity is selectively adsorbed. The drawbacks of the process are that it is not applicable to a multi-component system like sea/sub-soil brine. This process does not give any clue about the production of superior quality salt directly from sea/sub-soil brine in a solar salt works with reduced levels of trace elements detrimental for its industrial applications.
Reference may be made to U.S. Pat. No. 4,072,472 dated 7 Feb., 1978 on “Production of High purity salt from high sulphate salt deposits” by A. Lukes Jerome it is reported that subterranean salt deposit is solution mined, and the resulting calcium- and sulphate-contaminated brine is treated with soda ash to precipitate calcium compounds. After settling the slurry the clear brine is evaporated in a series of solar ponds to produce high-grade sodium chloride. This process is not economically feasible for large solar salt works where salt is produced from sea/sub-soil brines. Moreover, the process removes only calcium content from salt and other impurities are not eliminated.
Reference may be made to U.S. Pat. No. 3,647,396 dated 7 Mar., 1972 “Production of High Purity Salt”, H. W. Dewittie et al. have claimed to have developed a process for the recrystallization of sodium chloride in the form of high purity cubic crystals from a sodium chloride source containing calcium sulphate impurity by multi-effect evaporation preceded by treatment of the hot sodium chloride saturated brine by flocculants and settling, to cause the undissolved calcium sulphate particles and other suspended solids to agglomerate and settle out of the brine prior to recrystallization of sodium chloride eliminating the conventional requirement for filtering the hot brine. The main drawbacks of the process are that it involves recrystallization adopting hot extraction method which is expensive, time consuming and energy intensive. There is no mention of the utility of the said method for production of pure salt directly in solar salt works.
Reference may be made to Central Salt a Marine Chemicals Research Institute's Biennial reports 2000-2002 and 2002-2004 appearing in the Institute's website (www.csmcri.org) wherein the purification of saturated brine using ion selective resins and a nanofiltration process for the reduction of Ca and SO4 from sea water is reported. Both the processes have not addressed the methodologies for the elimination of trace elements from salt.
Reference may be made to U.S. Pat. No. 6,776,972 dated 17 Aug., 2004, “Recovery of Common Salt and Marine Chemicals from Brine” by CSIR has reported a process for the recovery of high purity solar salt from subsoil/sea brines employing the brine desulphatation process. The process is further integrated with the recovery of marine chemicals like potash and magnesia. The process reduces Ca and Mg impurities from salt as specified by chlor-alkali industries but the patent is silent about the removal of trace element impurities from salt.
Reference may be made to EP 1928569 B1 dated 16 Mar., 2011, “A cost-effective process for the preparation of solar salt having high purity and whiteness” by CSIR have claimed to have developed a cost effective process for the production of high purity solar salt (>99.5% purity on dry basis after heap washing) with improved whiteness and having highly reduced levels of calcium and sulphate impurities and also of heavy metal ions. Although the process as developed has many advantages in terms of ease of operation and eliminating the calcium and sulphate impurities to the minimum possible levels, magnesium impurities are not reduced in the same proportion as a result of which the Ca to Mg ratio in the salt is <1 and, consequently, fails to meet the preferred ratio of 2-4 desired in chlor-alkali industries. In addition, the elimination of trace element impurities especially bromide has not been addressed in this patent.
Reference may be made to U.S. Pat. No. 8,021,442 dated 20 Sep., 2011 entitled “An improved process of preparation of common salt of high purity from brines in solar salt pans”, by CSIR have reported a process for the production of high purity solar salt with desired ratio of Ca to Mg suitable for chlor-alkali manufacture. This patent is also silent about the alleviation of bromide impurities, which is a specific requirement of certain specialty chemical industries.
Reference may be made to the article “Design and Layout of Solar Salt Works” by B. S. Joshi et al. (Sixth International Symposium on Salt, 1983-Vol. II, Table 5, p 284) wherein it is reported that success of an ideal salt works depends mainly on the optimum design and layout. Maximum yield and higher purity of salt can be achieved by proper-layout. Based on the gross yearly and seasonal evaporation rate, percolation losses, initial density of brine or seawater, number of available days, evaporation rate of different density brines and expected production, empirical relationships between these parameters are worked out. These computations form a guideline whenever a new salt works is to be laid out or improvement is sought in an existing salt works. To derive an empirical formula to calculate the area, the following factual data of the Bhavnagar Salt Works is considered for a model salt works with a production capacity of 5000 TPA.
CompartmentDensity RangeArea (m2)Crystallizer25-29° Be′38510Condensers23-25° Be′466517-23° Be′1944514-17° Be′1489210-14° Be′32112 6-10° Be′70930Reservoirs 3.5-6° Be′121395 3-3.5° Be′47664 2.5-3° Be′66303 2-2.5° Be′98858TOTAL514774
This paper also describes that as the brine is lost continuously due to percolation, makeup brine must be provided to achieve the targeted production of 5000 TPA. To feed the required quantity of saturated brine to the crystallizers, an additional quantity of brine equivalent to percolated (lost) brine has to be made up in order to maintain production.
It is evident from the prior art that there are certain drawbacks in all the processes, especially as applicable to solar salt production with reduced levels of trace elements in the field. There are many processes where a salt of high purity is obtained but the processes are either cumbersome or costly and not practical for implementation in the field. On the other hand, the process of recrystallization to produce high purity salt as reported in the recent prior art is an attractive process for production of high purity solar salt in the field, with reduced levels of Ca, Mg, SO4 and trace elements for its specialty industrial applications.